As the need for increased data storage changes, the search for higher density, faster access memory technologies also increases. One of these, holographic data storage, provides the promise for increased access to higher density data. The techniques for realizing such storage typically utilize some type of storage media, such as photorefractive crystals or photopolymer layers, to store 3-D "stacks" of data in the form of pages of data. Typically, coherent light beams from lasers are utilized to perform the addressing, writing and reading of the data from the storage media by directing these beams at a specific region on the surface of the media. Writing is achieved by remembering the interference pattern formed by these beams at this region. Reading is achieved by detecting a reconstructed light beam as it exits the storage media, the data then being extracted therefrom. Addressing is achieved by the positioning of the laser beams, and this is typically done through the mechanical movement of mirrors or lenses; however, the storage media itself can be moved relative to fixed laser beams.
One storage media that is utilized for storage of information in the form of an interference pattern is a photopolymer material. This material is typically utilized to form conventional holograms. When initially fabricated, this photopolymer material is comprised of a thin layer of photopolymer disposed on some type of transparent substrate. A capping layer of a material, such as Mylar.RTM., is then disposed over the photopolymer layer. Initially, the photopolymer layer will have a large amount of unpolymerized monomers, a large amount of dye and initiators. The normal polymerization procedure is to irradiate the photopolymer with photons which will then begin the polymerization process. The reaction sequence associated with this process is complex. A simplified, but reasonably good model is as follows: the dye is first exited by photons and then the excited dye transfers energy to the initiator to provide an exited initiator. The excited initiator then combines with a monomer, which begins a chain reaction where initiator combinations combine with additional monomers to result in larger combinations, and so on, yielding a polymer. However, when oxygen is present in the photopolymer material, the excited initiator, instead of combining with the monomer, will combine with the oxygen, resulting in a termination step. If there is sufficient oxygen in the photopolymer material, the polymerization procedure will not rise to an acceptable level for a record operation until the available oxygen is consumed by initiators. This presents a problem in that oxygen is a relatively common molecule in the environment in which the photopolymer exists and readily diffuses through the photopolymer material. Therefore, if a particular region of the photopolymer is irradiated to a sufficient extent to remove the oxygen through combination with excited initiators, oxygen from an adjacent region of the photopolymer could diffuse into the irradiated region at a later time, thus requiring oxygen to be removed each time a recording is made in a particular region.
The ideal holographic storage media is inert to light until such time as one wishes to record, and then it becomes quite sensitive. Ideally, something simple can be done to the media at this time to make it sensitive. The oxygen in the photopolymer provides such a mechanism, and if used innovatively, can make photopolymer an ideal storage media.